Abstract

We show that an optical vortex beam, implemented classically, can be transferred to the transverse amplitude of a heralded single photon. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, using a pump in the form of a Bessel-Gauss (BG) beam with orbital angular momentum (specifically, with topological charge l = 1 and l = 2). We have designed our source so that it operates within the short SPDC crystal regime for which, the amplitude and phase of the pump may be transferred to a heralded single photon. In order to verify the vortex nature of our heralded single photon, we have shown that the conditional angular spectrum and the transverse intensity at the single-photon level match similar measurements carried out for the pump. In addition, we have shown that when our heralded single photon is diffracted through a triangular aperture, the far-field single-photon transverse intensity exhibits the expected triangular arrangement of intensity lobes associated with the presence of orbital angular momentum.

Figures (6)

(a) Parameter space (crystal L and parameter kt), with the region where the short crystal approximation is valid shaded in yellow. While point A corresponds to the experimental conditions (see below), point B lies outside of the short-crystal regime. (b) Simulated CAS for configuration A. (c) Simulated CAS for configuration B. (d) Assumed pump angular spectrum. It may be seen that while the CAS for configuration A very nearly matches the pump angular spectrum, the CAS for configuration B differs substantially from the pump angular spectrum.

Back: Bessel-Gauss beam preparation. Front: Experimental setup for the transfer of optical vortices with l = 1 and l = 2 topological charge from a pump beam to a heralded single photon; note that while all measurements use a spatially-resolving detector on FP1, a second detector is either not present (for the SPDC angular spectrum), or placed on any of the planes FP1, FP2 and FP3 for measurements of the CAS, transverse intensity and far-field diffraction pattern, respectively. Inset: SPDC and pump angular spectrum on plane FP1, where we have marked the location of the idler detector and the CAS; the dotted square represents the area where the signal detector is scanned.